The optical waveguide structures are not limited to any particular type of waveguide, i.e. the invention can for example be applied to a diffused or embedded waveguide (FIG. 1a), a raised stripe waveguide (FIG. 1b), a rib waveguide or optical strip-line (FIG. 1c), a general channel waveguide (FIG. 1d) or a ridge waveguide (FIG. 1e).
Furthermore, the optical waveguide structures are not limited to any particular longitudinal geometry, i.e. any kind of straight or curved geometry as used in Y-junctions, polarising beam splitters etc. can be used. FIG. 2 shows examples of such basic structures: FIG. 2a: Y-junction, FIG. 2b: WDM-device; FIG. 2c: star coupler and FIG. 2d: polarising beam coupler.
The diffused channel waveguide of FIG. 1a comprises for example a substrate material of LiNbO.sub.3 with a waveguide made by titanium indiffusion. Due to the imperfections during the fabrication processes used for making the waveguides in FIG. 1 (for example disuniformities in the titanium stripe dimensions, temperature gradients during diffusion, etc.) the effective waveguide birefringence varies locally over the wafer used for making a plurality of such devices at the same time and also as an averaged value from wafer to wafer. The performance of single optical components (e.g. straight and curved waveguides) as well as more complex integrated optical devices, like an acousto-optical mode converter depends critically on the uniformity of the waveguide birefringence. Thus, the overall performance and reproducibility of acousto-optical devices strongly depends on the homogeneity and reproducibility of the fabrication processes.
Birefringence essentially means that the effective index (or the propagation constant) for (quasi) TE-modes and TM-modes is different and therefore the requirement of a small variation of birefringence means that the difference in propagation constants or the difference in refractive index .DELTA. n remains the same along the optical waveguide as much as possible.
The birefringence variations can have detrimental effects even in simple single waveguides. In integrated optics and also in distributed optical communication systems it is often desirable to switch the input polarisation of a TE-mode to the TM-polarisation and this can, for example, be performed by electro-optical couplers or by an acousto-optical mode converter. The latter device is based on the usage of a birefringent optical waveguide and if this waveguide has birefringent variations this will cause the performance of this device to deteriorate drastically.
The detrimental effects of birefringent variation in the basic acousto-optical mode converter are explained with reference to FIG. 3. The working principle of an integrated acousto-optical device e.g. on LiNbO.sub.3 is based on a wavelength selective polarisation conversion between two co-propagating optical waves polarised along the main birefringence axes of the LiNbO.sub.3 -crystal i.e. between the"TM"- and "TE"-modes. Energy can be exchanged between these orthogonal polarisation modes when they get coupled by the off-diagonal elements in the dielectric tensor. This is possible for example by the electro-optic or photo-elastic effect as explained below. A surface acoustic wave, i.e. an elastic "Rayleigh-wave" in a photoelastic and piezoelectric material such as in LiNbO.sub.3 is an ideal means of coupling due to its tunability in frequency and in power.
As shown in FIG. 3 a straight monomodal waveguide of conventionally for example 7 .mu.m is embedded in about a 100 .mu.m wide monomodal acoustic-waveguide (x-cut, y-propagating LiNbO.sub.3 -crystal). Both optical waveguides and acoustic claddings are fabricated by a titanium indiffusion. Metal-interdigital transducers of a suitable configuration are deposited on top of the crystal at the beginning of the acoustic waveguide. By applying a RF-drive signal at the interdigital transducer electrode an acoustic wave is excited. The acoustic wave travelling along the interaction length induces the mode coupling for the optical polarisation modes. To define a certain conversion band width, the interaction length L is limited by an acoustic absorber.
A fundamental condition for energy transfer is the phase matching between the polarisation modes which results from the solution of the coupled wave equations. A conversion efficiency of 100% can only be achieved if the phase difference between the two optical modes (TE- and TM-modes) with different effective refractive indices is continuously compensated, which means a completely synchronous interaction along the interaction length. This synchronous interaction is essentially caused by means of an acoustic "Bragg"-grating having a pre-determined period and inducing a coupling between the "TE"- and "TM"-mode. The coupling effect is described by the following equation: ##EQU1##
Here n.sub.eff,TM and n.sub.eff,TF are the effective refractive indices for the (quasi) TE- and TM-modes, .beta..sub.TM, .beta..sub.TE are the propagation constants for the wavelength .lambda. (in vacuum) and .LAMBDA..sub.ac is the wavelength of the acoustic wave (i.e. the periodicity of the perturbation of the dielectric tensor induced for instance by a periodic electric field or a surface corrugation, i.e. the acoustic "Bragg"-grating. Typically, the .LAMBDA..sub.ac is about 20-21 .mu.m for .lambda.=1530-1570 mm. The propagation constant (wavenumber K.sub.ac) is ##EQU2##
where .LAMBDA..sub.ac is the acoustic wavelength, f.sub.ac is the frequency and v.sub.ac is the velocity of the acoustic wave. This is a phase matched (and thus wavelength dependent) process and a variation of the waveguide birefringence has a drastic effect on the phase matching and thus negatively influences the spectral conversion characteristics. The longer the waveguide is, the more detrimental the variations of birefringence on the phase matching is.
For optical wavelengths which do not fulfil the phase matching conditions the deviation .delta. from the ideal phase match condition can be expressed by the following equation: ##EQU3##
where .DELTA.n.sub.eff is the difference between the effective refractive indices of the guided polarisation modes. At a fixed acoustic frequency f.sub.ac, the value .delta. is a function of the optical wavelength .lambda. and of .DELTA.n.sub.eff. Only for .delta.=0 a perfect phase matching exists and a complete energy transfer is possible. In a highly birefringent material as LiNbO.sub.3 (.DELTA.n.sub.eff.apprxeq.0.072) the phase mismatch .delta. is a relatively strong function of the wavelength and hence LiNbO.sub.3 is a good candidate to fabricate components with conversion characteristics of small bandwidths. However, variations in n.sub.eff,TE, n.sub.eff,TM (i.e. .DELTA.n.sub.eff) will influence whether or not a perfect phase matching can be achieved. The conversion efficiency .eta. in case of a phase matched interaction is given by: EQU .eta.=sin.sup.2 (.gamma.P.sub..infin. +L L) (4)
where the factor .gamma. contains the material constants contributing to the coupling (elastooptic and piezoelectrical coefficient), P.sub.ac is the power of the acoustic wave and L is the length of the acousto-optical mode converter. It is seen that the conversion efficiency .eta. is, for the example of a constant coupling, approximately in the form of a (sin x/x).sup.2 -type behaviour (acoustic directional couplers with weighted coupling may for example possess a Gaussian-type behavior--therefore, the constant coupling only serves as one illustrative example here).
FIG. 4 shows a typical experimental result of measuring the conversion efficiency of a LiNbO.sub.3 acousto-optical mode converter of the "constant coupling type" comprising a diffused waveguide of a width of 7.0 .mu.m, a length of 60 mm and a height of 1250 .ANG. (.ANG.=10.sup.-10 m). The values of width and height correspond to values before indiffusion. The graph in FIG. 4 shows many side lobes an does not very well resemble a (sin x/x).sup.2 function due to strong variations in the birefringence of the optical waveguide.
FIG. 5 (relating to the same device as in FIG. 4) shows the phase matching frequency along an acousto-optic mode converter of 6 cm length for light of a wavelength of 1536 nm. The phase matching frequency and the effective birefringence are proportional to each other (.DELTA.n.sub.eff =.lambda..multidot.f.sub.ac /v.sub.ac ; v.sub.ac =3780 m/s for LiNbO.sub.3 in the case of the above device). FIG. 5 illustrates that the type of optical waveguide used has a strong birefringence variation since the phase matching frequency alters along the longitudinal direction. Such experimental results can be obtained by using a pulse probing technique as is described in "Acoustic Pulse Measurements of Acousto-Optic Tunable Filter Properties" by L. B. Aronson, G. Ranken, T. R. Ranganath, D. W. Dolfi in (IPR'95, DANA POINT, post-deadline paper #6-2). Therefore, in order to improve the performance of any integrated optic device that uses an acoustic-optical mode converter, such as an acousto-optic tunable 2.times.2 wavelength space switch (FIG. 6) or an integrated acousto-optical filter (FIG. 7), the aim is to make the conversion efficiency (shown in FIG. 4) to correspond as much as possible with the ideal curve. In the case of a constant coupling this means to make the conversion efficiency to correspond with the (sin x/x).sup.2 -type behaviour. This is achieved by having the birefringence of the optical waveguide varying as little as possible along the optical waveguide (FIG. 5).